Freshwater of Isparta and Egirdir is supplied from the Egirdir Lake, which is the second largest freshwater lake of the Lakes District in Western Turkey. The Egirdir Lake has been studied within the framework of the Basin Protection Plan Special Provisions of the Egirdir Lake. The impact of runoff is taken into account in determining protection zones of the surface water reservoirs in Turkey. An approach that emphasizes the impact of groundwater flow in addition to the surface runoff has been adopted in this study. Water in Lake Egirdir is often classified as the Class II water according to terrestrial water resources quality criteria in Water Pollution Control Regulation of Turkey. The geological and hydrogeological studies reveal a significant amount of groundwater recharge into the Egirdir Lake through carbonate rocks and alluvial deposits outcropping in the basin, which is why Egirdir Lake still has a less contaminated water quality in spite of heavy pollutants. For this purpose, groundwater flow is prominently used in defining protection zones and surface runoff as well. The inner protection zone, which is defined as the 50-day travel time, and the outer protection zone, defined as the 400-day travel time, were estimated by infiltrometer and pumping tests in alluvium. Pumping tests results were used for the determination of hydraulic conductivities and groundwater levels for the determination of hydraulic gradients. Protection zones in karstic areas are based on the vulnerability map and large karstic springs.